An organic light emitting diode (OLED) is a light-emitting diode (LED) in which the emissive electroluminescent layer(s) is a film of or based mainly on organic compounds which emit light in response to an electric current. This layer of organic semiconductor material is situated between two electrodes in some cases. Generally, for example, at least one of these electrodes is transparent. OLEDs sometimes are used in television screens; computer monitors; small or portable system screens such as those found on mobile phones and PDAs; and/or the like. OLEDs may also sometimes be used in light sources for space illumination and in large-area light-emitting elements. OLED devices are described, for example, U.S. Pat. Nos. 7,663,311; 7,663,312; 7,662,663; 7,659,661; 7,629,741; 7,601,436, 2011/0193477, and 2009/0295283, the entire contents of all of which are hereby incorporated herein by reference.
A typical OLED comprises at least two organic layers—e.g., electron and hole transport layers—that are embedded between two electrodes. One electrode typically is made of a reflective metal. The other electrode typically is a transparent conductive layer supported by a glass substrate. The one electrode generally is the cathode, and the other electrode generally is the anode. Indium tin oxide (ITO), which is typically transparent, often is used at the front portion of the OLED as the anode.
FIG. 1 is an example cross-sectional view of a typical OLED. The OLED includes glass substrate 102, transparent conductive anode layer 104, organic layer 100, cathode layer 110 and cover glass 112. The organic light emission layer 100 emits light, and light is generated by processes known from conventional OLEDs when electrons and holes injected into the organic layer 100 from different sides recombine. The organic layer 100 may include multiple layers. For example, in certain example instances the organic layer 100 may include a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer, and an electron injection layer. An example shown in FIG. 1 illustrates the organic semiconductor layer 100 including a hole transport layer (HTL), and electron transport layer (ETL), and an emitting layer (EL), where the ETL and emitting layer may or may not be present in one layer.
When a voltage is applied to the electrodes 104 and 110, the charges start moving in the device under the influence of the electric field. Electrons leave the cathode, and holes move from the anode in opposite direction. For example, the recombination of these charges leads to the creation of photons with frequencies given by an energy gap between LUMO and HOMO levels of the emitting molecules, so that the electrical power applied to the electrodes is transformed into light. Different materials and/or dopants may be used to generate different colors, with the colors being combinable to achieve yet additional colors.
This disclosure relates to a design and method of making a transparent conductive electrode (e.g., anode) on the light-emitting side of the organic layer of an OLED. For example, referring to the OLED in FIG. 1, this disclosure relates to an improved electrode 104 on the light emitting side of the organic layer 100 and a method of making the same. The electrode may be based on ITO. A second thin metallic or substantially metallic layer may be deposited on top of the first layer of or including ITO. The first and second layers are deposited (directly or indirectly) on the substrate, and may be deposited for example via sputtering at approximately room temperature. The thin metallic or substantially metallic layer may be of a single metal (e.g., Ni, Pt, or Au), a mixture of Ni, Pt, and/or Au, or may be a metallic or substantially metallic alloy (e.g., NiCrMo, NiCrAlFe, NiTi, NiMo, or mixtures thereof). After the thin layer is deposited over the TCO, the substrate (e.g., glass or quartz substrate) with the TCO layer and the thin layer thereon is subject to heat treatment (HT) in order to (a) thermally activate at least the TCO layer for desired electrical and/or optical properties, (b) increase the work function (WF) of the electrode, and/or (c) increase visible transmission of the electrode. The thin film over the TCO is advantageous in that it (i) serves as an oxygen blocking layer during subsequent thermal activation of the ITO to reduce undesired excessive oxidation of the ITO during its thermal activation, and/or (ii) controls the Fermi level of the transparent electrode to minimize or reduce the electrical barrier with the Highest-Occupied-Molecular Orbital (HOMO) of the Hole Transport Layer (HTL) of the OLED.
The first TCO layer (e.g., of or including ITO) is substantially more oxided as deposited than is the second layer which is a thin metallic or substantially metallic layer. The thin metallic or substantially metallic layer may be either non-oxided or slightly oxided in different embodiments. In certain example embodiments, as deposited, the first TCO layer contains at least about 15% more oxygen, more preferably at least about 20% more oxygen, even more preferably at least about 30% more oxygen, and most preferable at least about 40% more oxygen, than does the subsequently deposited thin metallic or substantially metallic layer.
In certain example embodiments of this invention, there is provided method of making an organic light emitting diode (OLED) device, the method comprising: sputter-depositing a first layer comprising indium tin oxide (ITO) on a glass substrate; sputter-depositing a second metallic or substantially metallic layer on the glass substrate over and directly contacting the first layer comprising ITO to form an electrode structure, so that the first layer comprising ITO is located between at least the substrate and the second metallic or substantially metallic layer; heat treating the electrode structure including the substrate, the first layer comprising ITO, and the second metallic or substantially metallic layer, at temperature(s) of at least about 300 degrees C. in order to thermally activate the first layer comprising ITO; and providing the electrode structure in an OLED device so that an organic light emitting semiconductor layer is located between said electrode structure and another electrode. The sputter-depositing of the first and second layers may be performed at approximately room temperature.
In certain example embodiments of this invention, there is provided an OLED comprising: a transparent conductive electrode structure comprising a first layer comprising ITO and a second conductive layer on a substrate, the first layer comprising ITO being located between at least the substrate and the conductive layer; wherein the second conductive layer comprises one or more of Ni, Pt, Au, NiCrMo, NiCrAlFe, NiTi, NiCr, and NiMo; an organic light emitting layer located between said transparent conductive electrode structure and another electrode, and wherein said transparent conductive electrode structure is on a light emitting side of the organic light emitting layer.
In certain embodiments of this invention, there is provided an organic light emitting diode (OLED) comprising: a transparent conductive electrode structure comprising a first layer comprising indium tin oxide (ITO) and a second conductive layer on a substrate, the first layer comprising ITO being located between at least the substrate and the conductive layer; the first layer comprising ITO is thicker than the second metallic or substantially metallic layer, and the second metallic or substantially metallic layer has a work function of at least 4.5, more preferably of at least 4.6, and even more preferably of at least 4.7; and an organic light emitting layer located between said transparent conductive electrode structure and another electrode, and wherein said transparent conductive electrode structure is on a light emitting side of the organic light emitting layer.
These and other embodiments, features, aspect, and advantages may be combined in any suitable combination or sub-combination to produce yet further embodiments.